Gallium nitride substrate and optical device using the same

ABSTRACT

A gallium nitride substrate that a GaLα/CKα peak intensity ratio in EDX spectrum is not less than 2. The EDX spectrum is obtained in energy dispersive X-ray microanalysis (EDX) of a surface of the gallium nitride substrate using a scanning electron microscope (SEM) at an accelerating voltage of 3 kV.

The present application is based on Japanese patent application No.2012-047203 filed on Mar. 2, 2012, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a gallium nitride substrate and an opticaldevice using the gallium nitride substrate.

2. Description of the Related Art

GaN-based semiconductor crystals such as gallium nitride (GaN) hasattracted attention as a material of optical devices such as lightemitting diode (LED) which emits high-intensity blue light or long-lifelaser diode (LD) which emits blue light.

Bulk crystal growth of the GaN-based semiconductor crystals isdifficult, and accordingly, it is difficult to produce a large singlecrystal GaN with high quality. However, in recent years, a method ofmanufacturing a GaN-based semiconductor crystal has been proposed, usinga DEEP (Dislocation Elimination by the Epi-growth withInverted-Pyramidal Pits) method or a VAS (Void-Assisted Separation)method, etc., and also using GaN free-standing substrate in which a GaNsingle crystal is grown on a heterogeneous substrate by a HVPE (HydrideVapor Phase Epitaxy) method.

In the DEEP method, a patterned mask of SiN, etc., is formed on a GaAssubstrate which is removable by etching, a GaN layer is the formedthereon, plural pits surrounded by facet planes are intentionally formedon a crystal surface and dislocations are accumulated at a bottom of thepits to reduce dislocation in other regions. The GaAs substrate is thenremoved, thereby obtaining a GaN free-standing substrate with reduceddislocation (see, e.g., JP-A-2003-165799).

In the VAS method, a GaN layer is grown on a substrate of sapphire,etc., via a GaN substrate with voids and a TiN thin film having a meshstructure, thereby allowing separation of the GaN substrate andreduction of dislocation at the same time (see, e.g., JP-A-2004-269313).

The GaN free-standing substrate obtained by the above-mentioned methodsis flattened by grinding and polishing front and back surfaces of asubstrate epitaxially grown by the HVPE method. Subsequently, an outerperiphery of the substrate is shaped in order to have a circular shapewith a given diameter. Then, after removing processing strain bywet-etching, etc., the substrate is cleaned and a GaN mirror wafer isthus obtained.

A known method of polishing the GaN substrate is, e.g., disclosed inJP-A-2001-322899. In JP-A-2001-322899, after the GaN substrate is fixedto a substrate-attaching board using a wax, both front and back surfacesof the GaN substrate are polished by loose abrasive supplied onto thesurface plate. Diamond is used as the loose abrasive by taking intoconsideration hardness of the GaN substrate.

SUMMARY OF THE INVENTION

However, crystal quality of an epitaxial growth layer is poor is case ofepitaxial growth using the GaN substrate polished by the method ofJP-A-2001-322899 and an optical device using such a substrate has aproblem that emission intensity decreases, which causes failure and adecrease in a yield. As a result of intensive examination of thisproblem, it was found that the diamond used as the loose abrasive isembedded into and remains on the surface of the GaN substrate when theGaN substrate is polished and crystal quality of the epitaxial growthlayer deteriorates due to a carbon component of the diamond. It is alsofound that the wax used when polishing remains and the crystal qualitydeteriorates due to a carbon component of the wax.

Accordingly, it is an object of the invention to provide a galliumnitride substrate that an amount of residual carbon on a substratesurface is small. It is another object of the invention to provide anoptical device that is formed using the gallium nitride substrate andhas excellent emission intensity.

(1) According to one embodiment of the invention, a gallium nitridesubstrate wherein a GaLα/CKα peak intensity ratio in EDX spectrum is notless than 2, the EDX spectrum being obtained in energy dispersive X-raymicroanalysis (EDX) of a surface of the gallium nitride substrate usinga scanning electron microscope (SEM) at an accelerating voltage of 3 kV.

In the above embodiment (1) of the invention, the followingmodifications and changes can be made.

(i) The GaLα/CKα peak intensity ratio in the EDX spectrum is not lessthan 3.

(2) According to another embodiment of the invention, an optical devicecomprises:

a device structure formed on the gallium nitride substrate according tothe embodiment (1).

In the above embodiment (2) of the invention, the followingmodifications and changes can be made.

(ii) The GaLα/CKα peak intensity ratio in the EDX spectrum is not lessthan 3.

POINTS OF THE INVENTION

According to one embodiment of the invention, a gallium nitride(GaN)substrate is constructed so as to satisfy the GaLα/CKα peak intensityratio of not less than 2. Based on the GaLα/CKα peak intensity ratiowhich is calculated as an amount of C (carbon) with respect to Ga, anincrease or decrease in the amount of residual carbon can be determinedso as to evaluate the amount of residual carbon on the surface of theGaN substrate. By satisfying the above GaLα/CKα peak intensity ratio, itis possible to obtain the gallium nitride substrate with the reducedamount of residual carbon on the surface thereof. Thus, the crystallinequality of an epitaxial layer grown on the gallium nitride substrate canbe enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

Next, the present invention will be explained in more detail inconjunction with appended drawings, wherein:

FIG. 1 is a diagram illustrating EDX spectrum of a GaN substrate at anaccelerating voltage of 3 kV;

FIG. 2 is a diagram illustrating a correlative relationship between anaccelerating voltage (Eb) and electron penetration depth (Re);

FIG. 3 is a schematic cross sectional view showing an HVPE apparatus formanufacturing a gallium nitride substrate in an embodiment of thepresent invention; and

FIG. 4 is a schematic cross sectional view showing an optical device inthe embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

When a GaN substrate is polished, carbon derived from diamond or waxremains on the substrate surface, as described above. The residualcarbon deteriorates crystal quality of an epitaxial growth layer to beepitaxially grown. Therefore, the present inventors measured an amountof residual carbon on a substrate surface and intensively examined arelation between the amount of carbon and a decrease in emissionintensity of an optical device to be formed. In detail, the surface ofthe GaN substrate was measured in energy dispersive X-ray microanalysis(EDX) and an amount of carbon was calculated from a GaLα/CKα peakintensity ratio in the obtained EDX spectrum. Then, influence of theamount of carbon on an increase or decrease in emission intensity wasevaluated. As a result, it was found that, when the GaLα/CKα peakintensity ratio is greater than a predetermined value and the amount ofresidual carbon near the surface the GaN substrate decreases, goodcrystal quality of the epitaxial growth layer is obtained and also theemission intensity of the optical device can be improved, and theinvention was made based on the findings.

GaN Substrate

When a surface of a gallium nitride substrate (GaN substrate) in thepresent embodiment is measured by a scanning electron microscope (SEM)at an accelerating voltage of 3 kV in energy dispersive X-raymicroanalysis (EDX), a GaLα/CKα peak intensity ratio in EDX spectrumobtained by the EDX is not less than 2.

The SEM is a device for observing a surface profile of a sample byirradiating an electron beam, which is a focused electron beam(electron) emitted from an electron source (electron gun), and scanningthe surface of the sample to detect secondary electrons emitted from thesurface of the sample. In the EDX, a characteristic X-ray generated fromthe surface of the sample at the time of the electron-beam-scanning bythe SEM is measured to identify elements contained on the surface of thesample. In addition, the number of counts/second (peak intensity) of thecharacteristic X-ray with given energy is measured to evaluate thecontent of a specific element. The EDX spectrum obtained by the EDX isas shown in, e.g., FIG. 1 (EDX spectrum of a GaN substrate inbelow-described Examples at an accelerating voltage of 3 kV). In FIG. 1,the horizontal axis indicates energy of the characteristic X-ray and thevertical axis indicates the number of counts/second [cps (count persecond)] of the characteristic X-ray at such energy. From FIG. 1, anapproximate content of an element constituting the GaN substrate isunderstood from a level of peak (peak intensity). In the presentembodiment, in order to evaluate the amount of residual carbon on thesurface of the GaN substrate, an increase or decrease in the amount ofresidual carbon is determined based on a GaLα/CKα peak intensity ratiowhich is calculated as an amount of C (carbon) with respect to Ga. Here,in the GaN substrate of the present embodiment, the GaLα/CKα peakintensity ratio is not less than 2, which indicates that C with respectto Ga is not more than a predetermined ratio.

Meanwhile, in the SEM, the electron beam (electron) is focused at apredetermined accelerating voltage. Here, the electron penetrates deeperfrom the sample surface when the accelerating voltage is higher and thisallows information about a deeper region from the sample surface to beobtained. In other words, in the SEM, it is possible to obtaininformation about a region at a predetermined depth from the samplesurface by adjusting the accelerating voltage to control electronpenetration depth.

Here, a relation between an accelerating voltage of the SEM and a depthof the sample (GaN substrate) to be measured will be described. Theelectron penetration depth depends on an accelerating voltage ofirradiated electron and an atomic weight, atomic number and density of ameasurement sample, and is calculated from the following formula (I)(see, e.g., JPn. J. ApPl. Phys. Vol. 40 (2001) PP. 476-479).

$\begin{matrix}{{Re} = {\frac{0.0276\; A}{\rho \; Z^{0.889}}E_{b}^{1.67}\mspace{14mu} ({\mu m})}} & (1)\end{matrix}$

FIG. 2 shows a correlative relationship between an accelerating voltage(Eb) and electron penetration depth (Re) in the above formula (1) incase that a GaN substrate is measured. FIG. 2 shows that the electronpenetration depth increases with an increase in the acceleratingvoltage. In other words, the electron penetration depth is smaller(shallower) when the accelerating voltage is lower and this allowsinformation about a region closer to the sample surface to be obtained.For example, Re is 0.09 μm when Eb is 3 kV, and Re is 0.20 μm when Eb is5 kV.

It is preferable to scan at a low accelerating voltage in the presentembodiment since the amount of carbon on the surface of the GaNsubstrate is evaluated. In this regard, however, when the acceleratingvoltage is low, the number of detectable characteristic X-rays ofelements decreases and also intensity of the characteristic X-ray to bedetected is low, which results in that it takes very long time tomeasure. Therefore, in the present embodiment, the amount of carbon onthe surface of the GaN substrate is evaluated by EDX using the SEM at anaccelerating voltage of 3 kV.

In the gallium nitride substrate of the present embodiment, the GaLα/CKαpeak intensity ratio in the EDX spectrum is not less than 2 in energydispersive X-ray microanalysis (EDX) using the SEM at an acceleratingvoltage of 3 kV. By this configuration, it is possible to obtain thegallium nitride substrate in which the amount of residual carbon on thesurface thereof is small. Therefore, in case of crystal growth usingthis gallium nitride substrate as a base substrate, crystal quality ofthe epitaxial growth layer to be obtained can be improved.

In the above-mentioned gallium nitride substrate, the GaLα/CKα peakintensity ratio in the EDX spectrum is preferably not less than 3. Bysuch a configuration, it is possible to further reduce the amount ofresidual carbon on the surface of the gallium nitride substrate and itis thus possible to further improve the crystal quality of the epitaxialgrowth layer.

In an optical device formed using the gallium nitride substrate of thepresent embodiment, since the crystal quality of the epitaxial growthlayer to be crystal-grown is good, emission intensity is high.

Method of Manufacturing Gallium Nitride Substrate

A method of manufacturing such a gallium nitride substrate includes astep of forming a gallium nitride substrate (GaN substrate), a step ofgrinding/polishing the gallium nitride substrate, a step of boiling andcleaning the gallium nitride substrate at a predetermined temperatureand a step of wet-etching the gallium nitride substrate at apredetermined temperature. In the present embodiment, the GaN substrateis formed by the VAS method.

Firstly, a GaN base layer is grown on a sapphire substrate by a MOVPEmethod. A metal Ti thin film is deposited on the GaN base layer.Subsequently, by heat treatment in a mixture stream of ammonium andhydrogen gas, the metal Ti thin film is nitride to turn into a TiN thinfilm having a mesh structure and also the GaN base layer is etched toform voids thereon, thereby forming a void-containing substrate.

Following this, a GaN crystal is grown on the void-containing substrateby the hydride vapor phase epitaxy (HYPE) method using GaCl and NH₃ asraw materials. In the HVPE method, a crystal growth rate is high and itis possible to easily grow a thick GaN crystal film. For growing acrystal by the HVPE method, an HVPE apparatus as shown in FIG. 3 isused.

The HVPE apparatus has a reaction tube 12 and a heater 11 providedtherearound. The reaction tube 12 has a substrate holder 17 for placinga void-containing substrate 18, reaction gas inlet tubes 13 and 15opening near the void-containing substrate 18, an etching gas inlet tube14 opening near the void-containing substrate 18 and an exhaust outlet21. A raw material deposition chamber 20 having a Ga metal 16 therein isprovided on the reaction gas inlet tube 15.

NH₃ is supplied to the reaction gas inlet tube 13 and HCl gas issupplied to the reaction gas inlet tube 15. The reaction gases aresupplied together with a carrier gas such as H₂ or N₂. In the reactiongas inlet tube 15, the Ga metal 16 housed in the raw material depositionchamber 20 is reacted with HCl and GaCl is thereby produced. In otherwords, GaCl and NH₃ are supplied from the reaction gas inlet tubes 13and 15 to the void-containing substrate 18. GaCl is reacted with NH₃ anda GaN crystal is thereby vapor-grown on the void-containing substrate18. The HCl gas for etching is supplied from the etching gas inlet tube14 to the void-containing substrate 18. The HCl gas is suppliedcontinuously during a crystal growth process or is supplied betweencrystal growth processes in order to make individual initial nucleilarge.

The thick GaN film is naturally separated from the sapphire substrate atthe voids in the course of temperature drop after the crystal growth andthe GaN substrate (GaN free-standing substrate) is thereby obtained.

Subsequently, the GaN substrate is attached and fixed to a ceramic plateby a wax and the back surface of the GaN substrate is ground/polished toimprove flatness of the GaN substrate. Likewise, the front surface(growth face) of the GaN substrate is ground/polished. Diamond slurrywhich is abrasive grain is embedded into the surface of the GaNsubstrate in this process. Meanwhile, the wax is removed by heating butslightly remains on the surface of the substrate. In other words, acarbon component is attached to and remains on the surface of the GaNsubstrate in the grinding/polishing process.

Following this, the polished GaN substrate is boiled and cleaned at apredetermined temperature. The residual wax on the surface of the GaNsubstrate is removed by this cleaning process, thereby reducing thecarbon component on the substrate surface. Temperature for boiling andcleaning is preferably not less than 40° C. By setting the temperatureto not less than 40° C., reactivity of a cleaning agent used is improvedand it is possible to dissolve the wax containing the carbon componentand thus to enhance removal thereof. In other words, it is possible toappropriately remove the carbon component and thus to increase theGaLα/CKα peak intensity ratio in the EDX spectrum. The cleaning agent tobe used is not specifically limited but is preferably isopropyl alcohol(IPA) which can appropriately remove the carbon component derived fromthe wax.

Furthermore, the polished GaN substrate is wet-etched at a predeterminedtemperature. Processing strain on the GaN substrate is removed by thewet-etching process. In addition, the residual wax which could not becompletely removed in the cleaning process is removed, together with thediamond slurry embedded into the surface of the GaN substrate, by thewet-etching process, thereby reducing the amount of residual carbon onthe substrate surface. In the etching process, the etching is preferablycarried out at not less than 77° C. by heating etchant. Etching at arelatively high temperature improves etching reactivity and thus allowsetching treatment time to be shortened. In addition, it is possible todissolve the wax and thus to appropriately remove the carbon component.

Method of Manufacturing Optical Device

Next, a method of manufacturing an optical device in which the GaNsubstrate obtained as described above is used to manufacture the opticaldevice will be described.

A nitride semiconductor crystal such as InGaN is epitaxially grown onthe surface of the above GaN substrate by the MOVPE method. In thepresent embodiment, since the amount of the residual carbon component onthe surface of the GaN substrate is small, crystal quality of thenitride semiconductor crystal to be grown is good. In addition, goodcrystal quality provides high emission intensity, reduces failurescaused by a decrease in emission intensity and allows a yield to beimproved.

Although the gallium nitride substrate formed by the VAS method has beendescribed in the embodiment, the invention is not limited thereto and isapplicable to a gallium nitride substrate formed by the DEEP method,etc., in the same manner.

EXAMPLES

Gallium nitrides substrate and optical devices in Examples of theinvention were manufactured by the following method under the followingconditions. These Examples are the illustrative gallium nitridesubstrate and optical device of the invention and the invention is notlimited to these Examples.

Example 1

In Example 1, a GaN single crystal was grown by the VAS method to make aGaN substrate.

Firstly, a void-containing substrate was prepared. For making thevoid-containing substrate, a 500 nm-thick GaN base layer was formed on asapphire substrate (3.5 inches in diameter) by the MOVPE method, etc., a30 nm-thick Ti layer was deposited on a surface thereof, andsubsequently, heat treatment (at a temperature of 1000° C.) was carriedout in a mixture gas of H₂ and NH₃ for 30 minutes to form voids in theGaN layer while converting the Ti layer into TiN having a meshstructure.

The void-containing substrate was placed on the substrate holder 17 inthe HVPE apparatus shown in FIG. 3, and was heated in the reaction tube12 at atmospheric pressure so as to have a substrate temperature of1050° C. The initial nucleation conditions were as follows: 5×10⁻² atmof NH₃ gas was introduced together with 6×10⁻¹ atm of N₂ gas as acarrier gas from the reaction gas inlet tube 13, 5×10⁻³ atm of GaCl gaswas introduced together with 2.0×10⁻¹ atm of N₂ gas and 1.0×10⁻¹ atm ofH₂ gas as carrier gases from the reaction gas inlet tube 15 and acrystal was grown for 20 minutes.

After the initial nucleation, the crystal was grown under the sameconditions as the initial nucleation conditions except that the partialpressure of GaCl gas was set to be 1.5×10⁻² atm and the partial pressureof N₂ gas as the carrier gas of NH₃ gas was set to be 5.85×10⁻¹ atm. Thecrystal was then grown until the entire GaN crystal becomes 900 therebyobtaining the GaN crystal. The thick GaN film was naturally separatedfrom the sapphire substrate in the course of temperature drop after thegrowth of the GaN crystal, thereby obtaining a free-standing GaNsubstrate.

Subsequently, the surface of the GaN substrate was attached and fixed toa ceramic plate using a wax. After that, the back surface of the GaNsubstrate was ground by a horizontal surface grinding machine. Theconditions for grinding the back surface were as follows: grinding stoneused—metal bond #800; diameter of grinding stone—150 mm; rotation speedof grinding stone—2000 rpm; feeding speed of grinding stone—0.1μm/second; and grinding time—30 minutes. Furthermore, the back surfaceof the GaN substrate was polished by a high speed single-surfaceprecision lapping machine. The conditions for mechanical polishing ofN-polar surface were as follows: rotation speed of surface plate—200rpm; pressure—0.25 MPa; polishing solution—diamond slurry (looseabrasive) having a grain diameter of 3 μm; feed rate of polishingsolution—0.3 L/min; and polishing time—20 minutes. Then, the ceramicplate to which the GaN substrate is attached was heated by a hot plateto melt the wax, thereby separating the GaN substrate.

In addition, the front surface which is another surface of the GaNsubstrate was ground/polished in the same manner as the back surface.The grinding conditions were as follows: grinding stone used—metal bond#800; diameter of grinding stone—200 mm; rotation speed of grindingstone—2500 rpm; feeding speed of grinding stone—0.1 μm/second; andgrinding time—30 minutes. The polishing conditions were as follows:rotation speed of surface plate—200 rpm; pressure—0.30 MPa; polishingsolution—diamond slurry (loose abrasive) having a grain diameter of 1μm; feed rate of polishing solution—0.30 L/min; and polishing time—20minutes. The ground and polished GaN substrate then had a thickness of400 μm.

Subsequently, the outer diameter process was performed on the GaNsubstrate by an outer diameter processing machine so as to have adiameter of 76.2 mm (3 inches).

Next, for the purpose of removing the wax attached to the surface of theGaN substrate, the substrate was boiled and cleaned for 30 minutes usingWA (isopropyl alcohol). During the cleaning, the cleaning temperaturewas set to 41° C. In addition, for the purpose of removing processingstrain on the GaN substrate and the carbon component derived from thediamond slurry embedded into the substrate surface, wet-etching wascarried out by immersing the GaN substrate in a 25% NH₄OH solution. Thewet-etching was carried out for 90 minutes at an etching temperature of77° C. The cleaning condition and the wet-etching condition of the GaNsubstrate are shown in Table 1.

TABLE 1 IPA boiling-cleaning Wet-etching temperature temperature Example1 41 77 Example 2 44 78 Example 3 47 79 Example 4 50 80 Example 5 53 81Example 6 56 82 Example 7 59 83 Example 8 62 84 Example 9 65 85 Example10 68 86 Example 11 71 87 Example 12 74 88 Example 13 77 89 Example 1480 90 Comparative Example 1 20 70 Comparative Example 2 23 71Comparative Example 3 26 72 Comparative Example 4 29 73 ComparativeExample 5 32 74 Comparative Example 6 35 75 Comparative Example 7 38 76

Lastly, the GaN substrate was washed with pure water and was dried by anitrogen gun, thereby obtaining a GaN substrate of Example 1.

Examples 2 to 14 and Comparative Examples 1 to 7

GaN substrates in Examples 2 to 14 and Comparative Examples 1 to 7 weremade under the same conditions as Example 1 except that the cleaningcondition (cleaning temperature) and the wet-etching condition (etchingtemperature) of Example 1 were changed to those shown in Table 1.

EDX measurement was performed on the surfaces of the GaN substratesobtained in Examples 1 to 14 and Comparative Examples 1 to 7, and theamount of residual carbon on the surface of the GaN substrate was eachevaluated. In detail, using VE-9800S (manufactured by KEYENCECORPORATION) as a scanning electron microscope (SEM) and GENESIS2000(manufactured by EDAX Inc.) as an EDX spectrum detector, EDX spectrum atthe center of the GaN substrate was measured at a characteristic x-raytakeoff angle of 16.28°. The measurement was performed while changingthe accelerating voltage of the SEM from 3 kV, 5 kV to 8 kV. Theelectron penetration depths at respective accelerating voltagescalculated from the formula (I) are respectively 0.09 μm, 0.20 μm and0.45 μm. Then, in order to measure the amount of carbon near the surfaceof the GaN substrate, a ratio of GaLa peak intensity (about 1.100 keV)to CKα peak intensity (about 0.266 keV) in EDX spectrum was examined.The results thereof are shown in Table 2.

TABLE 2 GaLα/CKα in EDX spectrum Accelerating Accelerating Acceleratingvoltage voltage voltage 3 kV 5 kV 8 kV Example 1 2.0 31.1 41.3 Example 22.2 32.7 40.6 Example 3 2.4 31.7 39.7 Example 4 2.6 33.7 42.9 Example 52.8 32.4 38.1 Example 6 3.0 33.8 39.0 Example 7 4.3 32.7 41.5 Example 85.4 31.5 38.6 Example 9 7.0 33.6 39.0 Example 10 8.9 32.9 41.1 Example11 10.3 31.9 40.6 Example 12 11.5 32.8 39.9 Example 13 12.9 31.9 39.2Example 14 14.1 32.8 40.1 Comparative 0.8 33.2 39.7 Example 1Comparative 0.9 31.0 42.5 Example 2 Comparative 1.1 31.8 41.6 Example 3Comparative 1.3 32.0 38.4 Example 4 Comparative 1.5 32.5 40.6 Example 5Comparative 1.7 31.4 42.8 Example 6 Comparative 1.9 33.7 38.5 Example 7

From Table 2, it was confirmed that, when the accelerating voltage ofthe SEM is 3 kV, the GaLα/CKα, peak intensity ratio increases with anincrease in the IPA boiling-cleaning temperature and NH₄OH wet-etchingtemperature and carbon near the substrate surface is removed. On theother hand, when the accelerating voltage of the SEM was 5 kV and 8 kV,the GaLα/CKα peak intensity ratio in the EDX spectrum hardly changed.This is because the penetration depth of electron beam into the surfaceof the GaN substrate is too far and variation in carbon level near thesubstrate surface is not observed. Therefore, an appropriateaccelerating voltage is considered to be 3 kV in order to examine thevariation in carbon level near the surface.

Following this, optical devices were manufactured using the GaNsubstrates obtained in Examples and Comparative Examples, and crystalquality was evaluated by measuring emission intensity thereof.

A H₂ carrier gas, ammonium, trimethylgallium and trimethylindium weresupplied onto a Ga-polar surface (front surface) of the GaN substrate ata substrate temperature of 1020° C. by the MOVPE method, thereby growinga structure of the epitaxial film shown in FIG. 4. In detail, a GaNbuffer layer 2 (2500 nm in thickness), an InGaN barrier layer (about 8nm in thickness) and an InGaN well layer (about 5 nm in thickness) werealternately laminated six times on the GaN substrate 1 (400 μm inthickness) of Example 1, and a multiple quantum well layer 3 formed bygrowing the InGaN barrier layer (about 8 nm in thickness) and a GaN caplayer 4 (about 30 nm in thickness) were further laminated thereon,thereby making an optical device 10.

Photoluminescence peak intensity corresponding to a band gap of an InGaNquantum well layer at the center of the GaN substrate was measured onthe obtained optical device by a photoluminescence measurement systemRPM 2000 (manufactured by Accent). The photoluminescence measurementconditions were as follows: laser light source—He—Cd laser with awavelength of 325 nm; width of light receiving slit—0.1 mm; andmeasurement-wavelength range—367.9 nm to 432.4 nm. Emission intensity ofthe GaN substrate was examined. The examination results are shown inTable 3.

TABLE 3 Photoluminescence emission intensity Example 1 1.515 Example 21.547 Example 3 1.493 Example 4 1.525 Example 5 1.563 Example 6 3.152Example 7 3.045 Example 8 2.997 Example 9 3.078 Example 10 2.965 Example11 3.036 Example 12 3.058 Example 13 2.987 Example 14 3.015 ComparativeExample 1 0.543 Comparative Example 2 0.589 Comparative Example 3 0.478Comparative Example 4 0.552 Comparative Example 5 0.513 ComparativeExample 6 0.492 Comparative Example 7 0.524

According to Table 3, in the optical devices of Examples 1 to 5 in whicha peak intensity ratio is not less than 2 and less than 3, thephotoluminescence emission intensity is 1.493 to 1.563 Volt/mW. Inaddition, in the optical devices of Examples 6 to 14 in which a peakintensity ratio is not less than 3, the emission intensity is 2.965 to3.152 Volt/mW. On the other hand, in the optical devices of ComparativeExamples 1 to 7 in which a peak intensity ratio is less than 2, theemission intensity is 0.478 to 0.589 Volt/mW. In other words, emissionintensity is lower in Comparative Examples 1 to 7 than in Examples 1 to14.

In Comparative Examples 1 to 7, the GaN substrate, of which GaLα/CKαpeak intensity ratio at an accelerating voltage of 3 kV is less than 2and in which the amount of residual carbon on the surface is large, isused and it is thus considered that the crystal quality deteriorates atthe time of growing the nitride semiconductor crystal and emissionintensity decreases. In contrast, the GaN substrate in which the peakintensity ratio is not less than 2.0 is used in Examples 1 to 14 and itis thus considered that the amount of residual carbon component on thesurface is small and the crystal quality of the nitride semiconductorcrystal to be grown is good. Especially in Examples 6 to 14 in which thepeak intensity ratio is not less than 3.0, the amount of residual carbonis smaller and it is thus considered that the crystal quality of theepitaxial growth layer is better. As a result of having good crystalquality, a decrease in emission intensity was suppressed in the opticaldevices of Examples 1 to 14, thereby obtaining large emission intensity.

Although the invention has been described with respect to the specificembodiment for complete and clear disclosure, the appended claims arenot to be therefore limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. A gallium nitride substrate, wherein a GaLα/CKαpeak intensity ratio in EDX spectrum is not less than 2, the EDXspectrum being obtained in energy dispersive X-ray microanalysis (EDX)of a surface of the gallium nitride substrate using a scanning electronmicroscope (SEM) at an accelerating voltage of 3 kV.
 2. The galliumnitride substrate according to claim 1, wherein the GaLα/CKα peakintensity ratio in the EDX spectrum is not less than
 3. 3. An opticaldevice, comprising: a device structure formed on the gallium nitridesubstrate according to claim
 1. 4. The optical device according to claim3, wherein the GaLα/CKα peak intensity ratio in the EDX spectrum is notless than 3.